This invention relates to a molding composition and process. More specifically, in one aspect this invention relates to a composition of matter useful in rotational molding, in another aspect it relates to a method of rotational molding.
A new family of olefin polymerization catalysts exhibiting very high productivity and other desirable features has recently been developed. This new generation of catalysts is termed high productivity catalysts and the olefin polymers produced therewith are termed high productivity polymers. Generally, in high productivity polymers the catalyst residue is allowed to remain. Some high productivity catalysts, methods of producing the high productivity catalysts, high productivity polymers and polymerization processes are disclosed in U.S. Pat. Nos. 4,325,837, 4,326,988 and 4,363,746 the disclosures of which are incorporated herein by reference. A presently preferred polymerization process uses a catalyst precoated with polymerized olefin, e.g., ethylene, monomer in a particle form (slurry) system in a loop reactor.
It has also been desired to utilize the high productivity polymers in a wide variety of polyolefin article fabrication processes. One process of growing importance is rotational molding. However, the high productivity polymers currently produced tend to exhibit two drawbacks when used in a rotomolding process: (1) excessive bubble formation in the molded part and (2) reduced degree of crosslinking achieved by the added peroxide compound. Possible rotomolding processes useful for this invention are disclosed in U.S. Pat. Nos. 4,029,729 and 4,115,508 and are incorporated herein by reference. Large or thick hollow articles such as tanks, drums, containers, etc. can be manufactured by the rotational molding of polymers.
Various compounds, such as stabilizers or crosslinking agents, can be added to polymers used in molding processes. Each additive generally lends the polymer based molding composition certain desired physical properties such as high strength or resistivity to heat or light. Additives can, however, cause a formation of large bubbles in the molded article. The bubble so formed can extend to each surface of the article and cause pinholes or openings in the walls of the article. A molded article can thus be rendered unsuitable for certain uses, such as storage of fluids. Several compounds have been suggested in the art for addition to polymer-based molding compositions and molding processes to reduce the formation of bubbles. Many of these bubble inhibiting compounds adversely affect the molding process parameters, such as cure rate, cure time and the physical properties of the molded article, such as color, impact strength, and tensile strength.
It is, thus, one object of this invention to provide a polymer based composition suitable for molding processes.
Another object of this invention is to provide a molding process for molding articles having improved crosslinking characteristics.
Another object of this invention is to provide a molding process for molding articles having no bubbles or a minimum of bubbles.
Still another object of this invention is to provide a polymer based rotomolding composition which will have improved crosslinking and few or no bubbles, without adversely affecting molding process parameters nor the physical properties of the molded product.